There are any number of applications for a self-supporting mechanism which can effectively transport a load along a straight line path from one point to another without substantial deviation from the straight line path, whether vertically or horizontally. For example, pile drivers and rock drilling tools come to mind. Another such application might be in extending a load onto a shelf, such as a fork lift truck might be employed to do.
One such application requires horizontal translation of a security gate for a roadway and/or walkway to an opened and a closed position.
Swinging (hinged) gates are well known and are in common use. My U.S. Pat. No. 4,503,723 for a Gate Operator Apparatus teaches one electrical/mechanical system for opening and closing such a gate. Swinging gates have an inherent disadvantage in that they frequently utilize valuable real estate in the area swept by the opening/closing operation.
In an effort to reduce that real estate requirement, straight line sliding or rolling gates have been utilized. Typically, these gates roll on wheels which either contact the ground, are operated on a track laid on the ground, or on a side or overhead mounted track. While such gates essentially resolve the problem of occupying an excessive amount of real estate, there are other problems associated with their use.
Overhead track systems establish an inherent height limitation for traffic through the gate area. The systems which operate on ground level tracks are subject to damage by the vehicular traffic over the ground laid track, and the wheeled models, which operate on and are supported directly by the ground, are subject to wear and tear on the wheels and on the supporting ground surface. Side mounted tracks may not extend into a protected drive or walkway area since they would then impede foot or vehicular traffic. Therefore, such side mounted track systems must employ a cantilever approach to support the gate over the drive or walkway. Ground level tracks may be rendered inoperable by ice or rocks and are subject to damage by snowplows and other road maintenance equipment. Ground track systems generally require a hard, level surface for support of the track and it is often necessary to provide a foundation for that surface.
Furthermore, such systems are subject to failure in the structure which is required to provide the gate with upright support and linear travel over a predetermined line of translation. Typically, such support is connected to, and provided from, an adjacent wall or fence which cooperates with the gate in providing security to an enclosed area. Many fences and walls do not have the physical integrity required to provide reliable support for the mechanism of the apparatus. Cantilevered systems require, typically, that approximately fifty percent additional length (30 feet total for a 20 foot gate) be freely available along the adjacent wall or fence for the cantilever mechanism.
Another problem related to poor tracking is the inherent difficulty in making an accurate closing contact with the wall or fence which bounds the gate area. It is somewhat difficult to provide an accurate enough closing contact to assure that the various means for locking the gate will function reliably.
All such gates require some means of linear force for operation. Initially, that was accomplished by means of a person physically operating the gate by pushing on it. This required that a vehicle operator leave his vehicle to operate the gate. That problem has been overcome, typically, by means of a cable or chain drive mechanism operated remotely with an electric motor providing the moving force. Such systems are subject to extreme wear and tear due to exposure to the elements and there are readily apparent safety problems attendant to the use of a cable or chain drive mechanism. Further, such systems are inherently plagued with severe friction forces and relatively large capacity motors must be utilized to energize them. Thus, in case of power failure, it is impractical to provide power to these larger motors from a solar energy source which is inherently restricted in its ability to provide the high power levels required.
It is known to use a counter-weight for the purpose of balancing a load to reduce the input energy requirements for moving that load. Examples of such use include elevators, certain lift bridges, such as the bascule bridge, and movable railroad crossing arms. It is also known to use springs to counterbalance loads for the same purpose as a counter-weight. Springs, of course, provide a variable (non-constant) force, depending upon the degree of compression or expansion.
While the sliding gate problem has been emphasized, supra, it is clear that there are many other applications for linear motion apparatus where it is important or mandatory to keep the traveled load path clear of the apparatus when the load is withdrawn from that path.